As shown in Fig. 2, based on this two-timescale channel property, [1]} proposed a two-timescale channel estimation framework, where the two different pilot transmission strategies are respectively designed for estimating the large timescale channel \(\bf {G}\)  and the small timescale channels \({\bf {h}}_d\)  and \({\bf {h}}_r\) . Firstly, the high-dimensional channel \(\bf {G}\)  is estimated once in a large timescale by using the dual-link pilot transmission strategy proposed in [1]}. Although the pilot overhead required for estimating \(\bf {G}\)  is large due to its high dimension, such overhead is negligible from a long-time perspective. Then, based on the widely used uplink pilot transmission strategy, the low-dimensional channels \({\bf {h}}_d\)  and \({\bf {h}}_r\)  can be estimated before data transmission in a small timescale. Although these channels have to be estimated more frequently, the required pilot overhead is small due to their low dimensions. As a result, the average pilot overhead can be significantly reduced by exploiting the two-timescale channel property.
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